This invention relates to a method for burying a step in a semiconductor substrate.
In forming a device-isolation-portion of an LSI, it is a conventional method to fill dielectric material or the like in a recess so as to bury the recess by the etch-back process which makes use of flat coating by spin coating of resist. Trench isolation which is used in isolation between devices of high-density LSI was reported by Kurosawa et al. at the IEDM in 1981 [IEDM (International Electron Device Meeting) Tech. Dig., 384 (1981)]. The method of burying the isolation recess and field region disclosed in this report is described below while referring to FIG. 3.
A dielectric material 16 (SiO.sub.2 deposit film), which is used for burying a lower part 14 of a step part 10 in a semiconductor substrate 12 in which the step part 10 is formed, is deposited, and a photoresist film 20 is formed by photolithography nearly at the same height as a higher part 18 of the step part 10 in the lower part 14 (FIG. 3A). A trench 22 is left over only between the higher part 18 and lower part 14. In the step shown in FIG. 3B, an organic film 24 is flatly applied by spin coating process from above, and the trench 22 is buried and flattened by making use of the nature of coating of organic film. In the step shown in FIG. 3C, only the organic film 24 is etched by using plasma beam 26 until the dielectric film 16 on the higher part 18 is exposed. Afterwards, in the next step shown in FIG. 3D, the photoresist film 20 and dielectric film 16 are etched by plasma beam 28 nearly to the surface of the higher part 18 of Si substrate 12 nearly in the same etching speed. As a result, the surface is flattened almost completely. Thereafter, by making devices in the semiconductor substrate 12 with the higher part 18 exposed, an LSI circuit is composed.
When the LSI is thus formed, the following three problems may occur. These problems are explained below by referring to FIG. 4. As shown in FIG. 4A, when applying the organic film 24 by spin coating, a difference is caused in the flow resistance on the photoresist film 20 and dielectric film 16, and a discrepancy in thickness occurs near the boundary, and the step part 10 of substrate 12 is convex, and if this convex part principal plane is broad, an extreme unevenness occurs, which makes it extremely difficult to flatten completely. In FIG. 4B, since the organic film 24 applied on the photoresist film 20 contains solvent, the photoresist film 20 is dissolved, and uneven state is caused, too. Finally, the problem in the step of etching the organic film 24 in the process of FIG. 3C is given in FIG. 4C. Usually, the end point of etching is detected by emission of light from the principal component to make up the material to be etched, and in the process shown in FIG. 3C, the atoms to compose the organic film 24 and photoresist film 20 are carbons, and since carbons pop out when ending, it is impossible to detect the end point.